build: remove HAVE_MALLOC_USABLE_SIZE.

The malloc_usable_size use in the STR type actually makes
operations like string_extend substantially slower. It is
faster to store the allocated size locally.

Originally, on platforms that have malloc_usable_size,
we were able to use the word of memory reclaimed from
the string type to store a cached hash code. But that logic
was revereted in 2022, so there is no such benefit.

* configure (have_malloc_usable_size): Variable
removed. Test for the malloc_usable_size function
removed.
(HAVE_MALLOC_USABLE_SIZE, HAVE_MALLOC_NP): Do not define
these preprocessor symbols.

* lib.c (HAVE_MALLOC_NP_H): Do not test for this variale
to include <malloc_np.h>
(string-own, string, string_utf8, mkstring, mkustring,
string_extend, string_finish, string_set_code,
string_get_code, length_str): Eliminate #ifdefs
on HAVE_MALLOC_USABLE_SIZE.

* lib.h (struct wstring): Eliminate #ifdef on
MALLOC_USABLE_SIZE, so alloc member is unconditionally
defined on all platforms.

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